1. Field of the Invention
This invention relates to poly(vinyl chloride) (PVC) compositions, especially those suitable for injection molding, and to novel additives which modify the rheological properties of the PVC composition.
2. Description of the Prior Art
It is known that processing of rigid poly(vinyl chloride), that is, polymers containing at least 80 weight percent vinyl chloride units and containing little or no plasticizer, is extremely difficult to accomplish without the use of polymeric processing aids. Such processing aids when used at levels of from about 0.1 to about 10 parts per 100 parts of poly(vinyl chloride) (PVC), more usually from about 0.5 to about 10 phr (parts per 100 parts of PVC), cause the PVC to flux under heat into a thermoplastic leathery state on a mill roll or other similar mixer. The processing aids further allow the PVC to be processed in an extruder into a molten thermoplastic state without the need for high shear forces and temperatures. They further impart to the processed product smoother, uniform surfaces.
A thorough description of processing aids for PVC may be found in "Thermoplastic Polymer Additives, Theory and Practice," Edited by John T. Lutz, Jr., Marcel Deckker, Inc., New York and Basel, 1988 (chapter by R. P. Petrich and John T. Lutz, Jr.) and "History of Polymer Composites," Edited by R. B. Seymour and R. D. Deanin, VNU Science Press BV, Netherlands, 1987 (chapter by D. L. Dunkelberger).
Particularly useful as processing aids have been high molecular weight polymers which are predominately composed of units derived from methyl methacrylate, which have been commercially available for about 30 years. These additives are especially useful in that they do not detract from the important physical properties expected from the final processed PVC object, such as heat distortion temperature, tensile strength, and impact-resistance properties.
One deficiency that the acrylic processing aids of commerce have is that the high molecular weight polymers, even at the low levels used, either maintain or increase the melt viscosity of the resultant blend. This is especially undesirable in injection molding applications, where it is desired to have a melt of low viscosity for ease in mold-filling. Lowering of the molecular weight of the processing aid will decrease the melt viscosity of the blend, but sometimes at the sacrifice of the heat distortion temperature of the final processed object.
Similar effects, i.e., improved flow at the expense of lowered heat distortion temperature, are found with other non-polymeric additives, such as plasticizers, or when a lower molecular weight PVC or a PVC copolymer is used.
Several polymeric additives based on polymers of (meth)acrylate esters have been disclosed as flow improvers for PVC. Simple homopolymers and copolymers, although effective, may delay fusion of the PVC, which is undesirable for rapid processing of the blend and for avoidance of overheating of the blend. Further, such single-stage materials are generally soft, difficult to isolate from their preparative media, and difficult to blend with the PVC. Further, when the additive is immiscible with the PVC and of significantly differing melt viscosity at processing temperatures, orientation effects may be seen in injection molding, and delamination of the processed blend may occur.
Multistage structures, often known as core-shell polymers, usually specifically described as graft polymers, have been described as useful for this purpose. However, the combination of relatively high molecular weight for the various stages and the chemical combination of the stages (grafting) make such materials less suitable for flow improvement in PVC formulations based on low molecular weight PVC, such as those of potentially commercial interest for injection molding. Further, when the molecular weight of the first stage is decreased and the compatibility with the PVC lessened to produce an additive which does not increase the melt viscosity of the blend with PVC, then delay of fusion is noted.
There has thus been a long-felt need for an additive which will allow PVC to be processed in injection molding to useful objects which maintain the physical properties of a medium- to high-molecular weight PVC, such as heat distortion temperature and toughness, while being capable of being molded under commercially practical and safe conditions of temperature, time, and pressure, which will further produce uniform, non-delaminated blends with PVC, which does not detract from, and preferentially enhances, the impact strength of the PVC blend, and which produces minimal deleterious effects on the fusion properties of the blend.